We're still not entirely sure what causes ADHD — genetic and environment factors all seem to play a role, according to Psych Central, with investigations finding that children with ADHD had smaller volumes of "white matter" and thinner tissue in some areas of their brains. But there's something about ADHD that needs to garner more attention: its occurrence in adults. It's estimated that four percent of all American adults have ADHD, which means that the likelihood that you'll know one of these people is pretty high.

Full disclosure: my husband had ADHD as a kid, and still manifests some of the symptoms as an adult, though he's had years of intensive therapy to help him with the disorder. As we'll get into here, having ADHD as a child is often a determining factor for symptoms as an adult, though in some cases it also turns up in adults who had not been previously diagnosed, particularly among women.

Here are some of the signals that an adult in your life might have untreated ADHD, because it goes beyond just restlessness and an inability to concentrate.

1. They Are Easily Distracted

This is the most famous symptom of ADHD, in children or in adults, and it can manifest in many different ways. People with ADHD can find it incredibly difficult to stay put on one task, thought, project or life course; they'll often procrastinate, find it tricky to finish one thing at a time, deviate from plans and organizational strategies, miss deadlines, and/or switch jobs, relationships and situations frequently. WebMD outlines a whole host of areas in which this distraction issue can show itself, from difficulty working on group projects in work situations to serial relationships and marriages.

2. They Have Periods Of Hyperfocus

If you're not familiar with ADHD as a phenomenon, this may come as a surprise to you: though the distraction that sufferers experience gets the big press, the flip side of the disorder is, for many, the experience of radical "tunnel vision," or hyperfocus. In other words, people with ADHD sometimes struggle to focus and find that their attention switches rapidly between subjects, and sometimes experience a level of focus on one single thing that can feel almost too intense.

3. They're Very Disorganized

The Anxiety & Depression Association of America defines lack of organization as one of the key components of an adult ADHD diagnosis, but they don't necessarily mean it in the sense of a messy room or a pile of unsorted shoes. People with adult ADHD are more prone to chronic disorganization crises, like forgetting about meetings or believing they're scheduled on different days, or misplacing important objects. My husband, an academic at Oxford and possibly the least stupid person I know, has twice booked flights to and from the wrong place, and he's had treatment. People with adult ADHD do not do these things because they are dumb; they are just often not focused on the key important details of a situation (like location, time, etc).

4. They're Impulsive

As part and parcel of the attention maladjustment of adult ADHD sufferers, they're also more inclined to act impulsively than adults without the disorder. The Royal Society Of Psychiatrists calls this "spur of the moment" thinking, in which ADHD sufferers do things like interrupt conversations or make high-risk choices (speeding, for instance) without a sufficient contemplation of the consequences. Impulsivity alone isn't a guarantee of ADHD, but impulsive decisions matched with poor organizational skills can be a sign that one should talk to a professional about getting a diagnosis.

5. They're "Scattered" (Especially If They're Female)

There's a fascinating gender divide in the experience of ADHD among adults. An investigation by The Atlantic in 2013 found two very interesting things about ADHD among women: one, it takes a different form from male disorders, and two, it may be much more likely to be diagnosed when the sufferer is an adult. According to their research, female sufferers of ADHD are more likely to be "disorganized, scattered, forgetful, and introverted," and it's been proposed that the hormonal fluctuations of puberty may actually increase ADHD symptoms in girls. Boys, meanwhile, often see a quietening of their symptoms as they start puberty.